1. Field of the Invention
The present invention relates generally to packaging methods and apparatus, and is particularly concerned with an improved method and apparatus for sealing cartons or containers of thermoplastic coated paperboard material.
2. Description of the Prior Art
The present invention is particularly concerned with an apparatus and method for sealing cartons or containers of the type made of paperboard stock coated on both sides with a thermoplastic material such as polyethylene. The thermoplastic coating on the paperboard is utilized not only as moisture proofing material, but also serves as a heat and pressure sensitive adhesive which cooperates in sealing the closure elements of the container so as to make the container fluid tight when it is filled with milk or other contents, and sealed and closed. An example of this general type of container is disclosed in U.S. Pat. Nos. 3,120,089 and 3,910,014.
Until recently, the conventional manner of sealing thermoplastic coated paperboard cartons was to bring the layers of the end closures of the container into contact with each other, and by the application of heat, activate the thermoplastic coating to cause it to flow at the portions to be joined and sealed. When the thermoplastic material cools and sets, the layers are sealed and adhesively secured together. In addition to requiring the use of elaborate and expensive apparatus for generating the considerable amount of energy required, elaborate apparatus, methods and systems are also required to dissipate the heat from the plant in which the packaging operation is being carried out. Examples of this general type of packaging apparatus are shown in U.S. Pat. Nos. 3,002,328; 3,120,089; 3,166,994, 3,187,647; 3,239,995 and 3,309,841.
U.S. Pat. Nos. 3,905,280 of Sept. 16, 1975 and 3,910,014 of Oct. 7, 1975 disclose apparatus for sealing the ends of thermoplastic coated paperboard containers by vibration welding wherein mechanical energy is converted into localized heat energy to minimize the amount of heat and energy required, and to eliminate the necessity for expensive systems and methods for dissipating the excess heat. U.S. Pat. No. 3,526,792 discloses an ultrasonic transducer having a converter for converting electrical energy into mechanical energy, and a concentrating horn for concentrating the mechanical energy into vibrations at a desired frequency. In the above mentioned U.S. Pat. Nos. 3,905,280 and 3,910,014, the horn of the vibration welding apparatus vibrates at a predetermined frequency when energized, and cooperates with an anvil to clamp the layers of the thermoplastic coated paperboard between the working surface of the horn and the back-up or work surface of the anvil. When the thermoplastic coated paperboard layers are clamped between the work surfaces of the horn and anvil, the vibration of the horn creates frictional heat sufficient to activate the thermoplastic coating of the layers to cause the thermoplastic material of the layers to flow together. When the vibration is terminated, the thermoplastic will cool and set to seal the layers and secure the layers together throughout the area of the seal. Among the advantages of the utilization of vibration welding is that the heat generated by the vibration of the horn is concentrated in the area to be sealed, and little, if any, excess heat is required to be generated to perform the sealing operation.
There are two stages in the sealing operation of thermoplastic coated paperboard containers by vibration welding. These stages are: (1) the activation stage in which the friction of the vibrating horn generates heat sufficient to activate the thermoplastic coating to cause it to flow, and (2) the cooling stage wherein the layers remain clamped between the horn and anvil with the horn deenergized to permit the previously activated thermoplastic coating to cool and set to form the seal. Thus, a complete cycle of forming a seal between layers of a thermoplastic coated paperboard carton involves (1) bringing the working surfaces of the anvil and horn into opposed relationship on opposite sides of the layers to be sealed, (2) activating the thermoplastic coating of the layers between the work surface by frictional heat to cause the coating of the layers to flow together, (3) permitting the previously activated thermoplastic material to cool and set while still held between the working surfaces of the anvil and deenergized horn, and (4) separating the working surfaces of the anvil and horn to remove the carton layers after the seal has been formed. The time involved per cycle to date has been in the range of approximately one-half to two seconds, depending upon the thickness of the paperboard, the thickness of the thermoplastic coating, the number of layers, and the surface area to be sealed.
It has been conventional in a vibration welding operation of this type to pressure actuate the horn to start the horn vibrating; that is, as the working surface of the horn moves into contact with the layers to be welded together, the horn starts vibrating when a predetermined pressure on the working surface of the horn is sensed by the horn actuating controls.
The pressure over the entire area to be sealed must be maintained within predetermined limits. If the pressure is too low, the friction between the vibrating horn and the paperboard layers will be insufficient to generate enough heat to activate the thermoplastic coating to flow. If the pressure is excessive, burning will occur, that is, the thermoplastic coating will be removed at high pressure points to destroy the seal.
Another problem that can occur when there is excessive clamping pressure between the horn and anvil is that some vibration may be transmitted to the anvil. If this occurs, the friction between the horn and paperboard layers will be reduced because of the resulting reduction in the relative motion between the vibrating horn and the paperboard layers. Stated another way, excessive pressure will tend to cause vibration of the anvil with the horn so that the paperboard layers will tend to move with the horn instead of frictionally reacting against the vibrating horn. The reduction in friction will reduce the possibility of generating heat sufficient to activate the thermoplastic, or at least will extend the time required to activate the thermoplastic coating. The likelihood of this occurring increases as the mass of the anvil decreases; an anvil with a small mass relative to the horn will have a greater tendency to start vibrating than would be the case with an anvil having a relatively large mass.
A significant factor in determining the production rate is the amount of time required in the sealing operation which, as alluded to above, involves both an activation and cooling stage. When a carton is at the welding station, and the layers to be sealed are clamped between the horn and anvil, inadequate cooling time after deenergization of the horn with the layers remaining clamped between the horn and anvil increases the likelihood that the layers will separate before the thermoplastic coating sets, and thus prevent formation of a seal. In a high production packaging system, the available time for the sealing operation is determined primarily by the speed at which the cartons must pass through the welding cycle as determined by the speed of other machines in the total system. For example, the welding apparatus must be incorporated into a system including machines for feeding and erecting cartons from flat blank form, such as machines of the type disclosed in Allen U.S. Pat. No. 3,599,541 of Aug. 17, 1971 and in Kellogg U.S. application Ser. No. 489,880, filed July 19, 1974, now U.S. Pat. No. 3,937,131, and filling and closing apparatus such is disclosed in Braun U.S. Pat. No. 3,910,014.